Browsing by Author "Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland"

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    Aerosol CVD synthesis and applications of single-walled carbon nanotube thin films using spark-discharged produced catalyst
    (Aalto University, 2020) Ahmad, Saeed; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Structural controlled synthesis of single-walled carbon nanotubes (SWCNTs) have attracted a great deal of attention due to their widespread potential applications in electronics and photonics. Floating catalyst chemical vapor deposition (FC-CVD) being a dry and continuous method, is a highly promising technique for the scalable synthesis of SWCNTs. However, due to in-situ catalyst formation in all the conventional FC-CVD approaches, it is hard to get full control of number concentration, composition and size of nanoparticles. Hence, it hinders to investigate the effects of catalyst composition on morphology, yield and structure of SWCNTs. In this thesis, firstly we designed a novel rod-to-tube type spark discharge generator (R-T SDG) to produce ex-situ catalyst nanoparticles for the FC-CVD growth of SWCNTs. We utilized highly time-stable and uniform number size distributions of monometallic (Fe, Ni, Co) and bimetallic (Co-Fe, Co-Ni) catalyst particles for the synthesis of SWCNTs using ethylene as carbon source and 1050 deg C temperature. Optical characterizations revealed that as-grown SWCNTs have high-quality and their mean diameter is around 1 nm. The highest SWCNTs yield was obtained with Fe as a catalyst. From electron diffraction analysis, we observed that Co-Ni can produce comparatively narrower chirality and diameter distribution of SWCNTs. Secondly, we introduced sulfur in the FC-CVD reactor as a growth promoter for the fabrication of SWCNTs based transparent conducting films (TCFs). We systematically investigated the roles of sulfur on yield, morphology, and structure of SWCNTs. It was found that the yield of SWCNTs is largely dependent on amount of sulfur introduced into the FC-CVD reactor and catalyst composition. More importantly, the addition of an optimized amount of sulfur has enhanced approximately three times, the opto-electronic performance of SWCNT-TCFs, by increasing diameter and bundle length along with improving the quality of SWCNTs. The mean diameter of SWCNTs increased from 1 nm to 1.2 nm while the ratio of metallic nanotubes slightly increased from 39 % to 41 % with sulfur addition. Surprisingly, chirality determination of as-grown SWCNTs indicated that sulfur promotor has little influence on modulating the chirality of SWCNTs. Finally, we demonstrated that FC-CVD is a unique and versatile technique for the simultaneous growth of substrate-free 0D-fullerene, 1D-CNT and 2D-graphene. The formation of 0D-1D-2D carbon nanostructures were directly evidenced by lattice-resolved (scanning) transmission electron microscopy (STEM). We showed that the relative number density of graphene-nanoflakes can be tuned by optimizing the synthesis conditions. In addition, the as-synthesized hybrid films can be directly deposited on any surface at ambient temperature with an arbitrary thickness which offers a new route towards ultra-fast manufacturing and dry deposition of the hybrid structures.
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    Carbon dioxide-assisted synthesis of single-walled carbon nanotubes and their thin film properties
    (Aalto University, 2019) Liao, Yongping; Jiang, Hua, Dr., Aalto University, Department of Applied Physics, Finland; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Owing to their unique chemical and physical properties, single-walled carbon nanotubes (SWCNTs) have attracted increasing attentation in various fields. The properties of SWCNTs strongly depend on their chirality and geometry. Thus, to realize the applications in desired fields, it is of significant importance to tune the chirality and geometry of SWCNTs. The floating catalyst chemical vapor deposition (FC-CVD) method, as a dry and continuous process, has been widely used in academic and industrial fields. However, tuning the growth of SWCNTs in FC-CVD is a challenge. In this thesis, first of all, certain amounts of carbon dioxide (CO2) is introduced to tune the growth of SWCNTs in a FC-CVD reactor, where carbon monoxide (CO) is used as carbon source and ferrocene as catalyst precursor. We found that the SWCNT thin films display different colors with various CO2 concentration, specifically, a green and brown colors are observed . The optical absorption spectrum of the green film shows a distinct absorption peak in visible range. Further analyzing the chirality by electron diffraction reveals that the green film possesses a very narrow chirality distribution near armchair. Besides the chirality, we also studied the geometry, such as tube diameter, bundle length and diameter, modulated by CO2. The SWCNT diameter and bundle length are found to increase with CO2 concentration. In addition, the yield and bundle diameter are also affected by CO2. Accordingly, SWCNT thin film prepared with certain CO2 concetration displays a remarkably reduced sheet resistance. Therefore, employing CO2 offers new strategy to tune the chirality and geometry of SWCNTs in FC-CVD. Furthermore, we also deposited our aerosol SWCNTs on wafer-scale substrates by large-scale thermophoretic precipitator (TP), such SWCNT thin films display ideal uniformity and conductivity. The scale-up deposition method is applicable in industrial productions of SWCNT transparent conductive films. To futher reduce the sheet resistance of SWCNT thin films, we then deposited SWCNTs on graphene by TP. The SWCNTs/graphene hybrid film exhibits improved conductivity. By measuring the temperature dependent conductance, we found that due to the presence of graphene, the tunnelling barrier between tubes has been reduced, which enhances the carrier tunnelling efficiency and thus, improves the conductivity.
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    Enhancing Optoelectronic Performance of Randomly Oriented Single-Walled Carbon Nanotube Films
    (Aalto University, 2019) Tsapenko, Alexey P.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland; Nasibulin, Albert G., Prof., Skolkovo Institute of Science and Technology, Russia
    Randomly oriented single-walled carbon nanotubes (SWCNTs) in the form of films are a promising material for various optoelectronic and photonic applications, including actual flexible and stretchable, transparent and conductive electrodes. However, the optoelectronic performance of as-synthesized SWCNT films still needs to be improved in order to provide industry-required conductivity characteristics. In this thesis, several novel approaches are introduced to enhance the optoelectronic properties of the films by an adsorption doping technique. The first approach based on the creation of a hybrid graphene/SWCNT material allowed us to reduce the sheet resistance by introducing a large area π-stacking interaction between the carbon nanomaterials. The second one is devoted to the optimization of an adsorption doping technique with a correct selection of a dopant solvent in which the evaporation rate is the most important parameter to control the optoelectric properties of the SWCNT films. The final one presents an aerosol-assisted approach that focuses on a uniform, controllable, and reproducible doping which leads to fine-tuning of the SWCNT film work function and conductivity. Additionally, the optical properties of the as-synthesized and doped nanotubes were extensively studied using broad wavelength non-destructive spectroscopies and optical pumping with a terahertz probe. The first study contributes and confirms the observable spectral effects for free carriers described in the Drude conductivity model. The second one identifies strong negative photoconductivity in both pristine and doped nanotubes. As a result, each of these cases leads to the creation of the SWCNT films that exhibit superior properties. This opens up numerous breathtaking opportunities for today and upcoming devices.
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    Gas-phase synthesis of single-walled carbon nanotubes from liquid carbon source for transparent conducting film applications
    (Aalto University, 2019) Ding, Erxiong; Jiang, Hua, Dr., Aalto University, Department of Applied Physics, Finland; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Owing to the exceptional optoelectronic properties of single-walled carbon nanotubes (SWCNTs), the transparent conducting films (TCFs) incorporating SWCNTs have been applied in areas like solar cells, touch screens, organic light-emitting diodes, and thin-film transistors (TFTs). Particularly, the SWCNT TCFs on a polymer substrate can maintain their properties well under mechanical bending and stretching. Thus, high-yield production of SWCNTs with desired morphological and structural features for the fabrication of highly conductive TCFs is of significance for their scaled-up applications. As for the representative application of SWCNTs in TFTs, semiconducting-enriched nanotubes are preferable.  This dissertation focuses on the high-yield production of SWCNTs for conductive film applications and the synthesis of semiconducting-enriched SWCNTs (s-SWCNTs). A dedicatedly designed aerosol reactor was constructed for SWCNT synthesis using liquid hydrocarbons as the carbon source injected with a syringe pump. Ethanol was first selected as the carbon source to produce SWCNTs. We optimized the growth parameters including thiophene and ferrocene concentrations, the hydrogen flow rate, the temperature as well as the feeding rate of the precursor solution. Limiting the feeding rate reduces the sheet resistance of the SWCNT TCF to ca. 78 Ω/sq at 90% transmittance at 550 nm. The SWCNTs synthesized from ethanol have morphological features like a mean diameter of 2 nm, a mean bundle length of 28.4 μm, a mean bundle diameter of 5.3 nm, and the chiral structures are clustered around the armchair edge. The roles of sulfur were systematically investigated as well using a spark-discharge aerosol reactor for SWCNT synthesis. An optimal amount of sulfur was found to promote the growth of large-diameter and long SWCNTs with high yield and improved quality. Sulfur was proposed to assist the formation of active sites on the catalyst surface to enhance SWCNT growth. To further decrease the sheet resistance and simultaneously keep a high yield, toluene was appointed to be an alternative carbon source. By producing larger-diameter (mean diameter is 2.3 nm) SWCNTs and longer (mean bundle length is 41.4 μm) nanotube bundles, the sheet resistance of the SWCNT TCF was decreased to ca. 57 Ω/sq at 90% transmittance with a much higher yield than that in the ethanol case. The chirality map of the SWCNTs depicted from the electron diffraction results presents a bimodal distribution of the chiral angles. In addition, high-purity s-SWCNTs were also continuously produced with ethanol as the carbon source and methanol as a growth enhancer. The s-SWCNT purity determined from the optical absorption spectrum can be higher than 95% which is beneficial for the high-performance electronics.
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    On the limit of single-walled carbon nanotube random network conductivity
    (Aalto University, 2015) Mustonen, Kimmo; Nasibulin, Albert G., Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Single-walled carbon nanotubes (SWCNTs) are one of the most interesting emerging materials for practical applications. As transparent conductive films (TCFs) and thin film transistors (TFTs) they provide prospects for both improved flexibility and conductivity over established metal oxide and silicon-based materials. Technologically crucial performance optimizations, however, require a coherent picture how the SWCNT network properties, specifically sheet conductance, absorbance and spatial uniformity, emerge from individual nanotubes. Here, a new kind of floating catalyst approach based on a spark discharge generator (SDG) is presented for the synthesis of predominantly individual SWCNTs in the gas phase. In this process, Brownian diffusion is identified as the major cause behind nanotube gas-phase aggregation (bundling). This can be avoided by limiting the SWCNT number concentration down to ~105 cm-3, yielding a high fraction of 60-80 % of individual tubes on substrates. For mostly individual 3-4 μm long SWCNTs, the observed aggregation rate matches a mobility diameter of 20 nm. The synthesized tubes exhibit a pre-eminence of near-armchair chiralities, up to 70 % having chiral angles ≥20°, with an unconventionally high fraction of semiconducting tube species, 80 %, at a growth temperature of 750 °C. Furthermore, by optical and electrical characterization of networks fabricated from individual tubes and small diameter bundles, unambiguous experimental evidence of the detrimental nature of SWCNT bundling on TCF performance is found. The performance loss is explained to be due to gratuitous absorbance in large diameter bundles, without a compensating conductivity gain. An absorbance-conductance model is presented, assuming that the Beer-Lambert law applies independent of the TCFs’ internal geometry, whereas at room temperature a significant charge carrier transport is allowed only through metallic-metallic tube junctions. The maximum network conductivity is expected where the nanotube lengthwise resistances between the junctions become as large as the junction resistances, providing the ultimate performance limit for metallicity-mixed SWCNT networks of 80 Ω/☐ at 90 % transparency. For all-metallic and doped networks, the limit is expected at 25 Ω/☐. In correspondence, nitric acid treated TCFs fabricated using individual 4 μm long SWCNTs are demonstrated with a sheet resistance of 63 Ω/☐ at 90 % transparency. Finally, random-network TFTs fabricated from the individual tubes approach the uniformity of ideal computer-simulated systems. The TFTs exhibit On/Off current ratios between 104 and 106 and simultaneous charge carrier mobilities up to 100 cm2 V-1s-1 combined with a fabrication yield of >99%. The normalized On-current shows standard deviation of ~25%, showing unprecedently high uniformity for random network TFTs.
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    Synthesis and applications of single walled carbon nanotubes from ethylene as carbon source
    (Aalto University, 2019) Hussain, Aqeel; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Single walled carbon nanotubes (SWCNTs) as emerging material have great potential, due to their striking structural and electronic properties, for applications in thin film transistors (TFTs), flexible displays and energy conversion. Floating catalyst chemical vapour deposition (FCCVD) has been applied to synthesize SWCNTs from various carbon precursors. Ethylene, as one important carbon source, has been widely used and studied in substrate CVD. However, SWCNT synthesis from ethylene in FCCVD and understanding about the structure and types of the SWCNTs produced are lacking. This dissertation comprehensively explored the growth of SWCNTs in ethylene-FCCVD. Ethylene (0.75 ccm) was first time used as carbon source alone in FCCVD along with 40 ccm hydrogen and 300 ccm nitrogen as the carrier gas. The whole growth process is environmental friendly and economical for scale-up production of nanotubes. Ethylene based FCCVD produced 1.50 nm mean diameter nanotubes. Electron diffraction shows random chirality distribution of SWCNTs between the armchair and zigzag edge. Moreover, the structure of nanotubes was further tuned with the introduction of water as a promoter. Optimum water concentration of 115 ppm reduced the mean diameter of the tubes to 1.10 nm and extremely narrow chirality distributed tubes were synthesized. The accumulation of single chirality (9,8) approaches to 27% of the tubes. The further studies also showed water can influence the formation of nanoparticles (NPs) in the gas phase, which might be the reason for SWCNT structure regulation in FCCVD. Furthermore, the transparent conducting films (TCFs) of SWCNTs were fabricated, based on ethylene-FCCVD. The TCFs consisting of highly individual SWCNT with length up to 13 μm, possesses high conductivity with sheet resistance as low as 51 Ohm/sq. for 90% transmission, which is one of the lowest sheet resistance value reported for SWCNT TCFs. In comparison, TCFs were also produced with same process, but with CO as carbon source. Due to bundling and short nanotubes, the SWCNT TCFs from CO has a higher sheet resistance of 89 Ohm/sq. for 90% transmission. Specifically, simultaneous growth of SWCNT and graphene was observed in ethylene-based FCCVD. It is believed that oversupply of carbon from ethylene decomposition assists the growth of graphene. In addition, the novel hybrid structure of 1D SWCNTs and 2D graphene was also fabricated. The interface of nanotubes and graphene hybrid showed carbon-carbon alignment spanning hundreds of nanometers.
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    Transparent, conductive and flexible single-walled carbon nanotube films
    (Aalto University, 2013) Kaskela, Antti; Nasibulin, Albert G., Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Nano Materials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Single-walled carbon nanotube (SWCNT) networks have a large application potential for future electronics as transparent conductive films. SWCNT networks (SWCNT-N) offer improved flexibility when compared to the current industry standard transparent conductive films (TCF), an example of which is indium tin oxide (ITO). SWCNTs can be synthesised from abundant raw materials, whereas indium supply is limited and has been a target of aggressive trade policies, thus increasing supply risks and price volatility. In order to make the SWCNT-Ns suitable for industrial applications, their performance must be made competitive with ITO and other TCF materials, whilst their manufacturing costs have to be minimised. Understanding the performance limiting factors is important when it comes to the development of high performance SWCNT networks. The results presented here show that the bundle length has a major impact on the electrical performance of SWCNT networks. Optimisation of SWCNT growth conditions in aerosol-CVD reactors used for SWCNT synthesis led to an increase in SWCNT bundle length from 1.3 μm to 9.4 μm. Bundle diameter distributions were found to overlap, with mean bundle diameter measuring approximately 10 nm, and mean SWCNT diameters ranging from 1.4 to 1.7 nm. The increased bundle length led to a reduction in the number of the highly resistive bundle-bundle contacts and to improved performance. When the SWCNT-TCFs were chemically doped by nitric acid, sheet resistance was reduced down to 84 Ω/sq. at 90% transparency, thus making the SWCNT TCFs competitive with ITO on polymer films. The intertube and interbundle contact resistances together with the effect of nitric acid treatment were studied by using conductive atomic force microscopy. The contact resistance values of pristine junctions were within the range of 29 kΩ - 532 kΩ for contacts between individual tubes and small bundles with less than 5 nm diameter. The contact resistance decreased with increasing tube or bundle diameter. Contact morphology had a major impact on the contact resistance values as X- contacts exhibited higher mean contact resistance of 180 kΩ, while the Y-contacts had mean contact resistance of 60 kΩ. When the contacts were exposed to strong nitric acid, the mean contact resistance was reduced by a factor of 3, although the length resistivity remained largely unchanged at around 8 kΩ/μm. The results indicate that the contact morphology and the diameter of contacting SWCNTs and bundles had a significant impact on the electrical transport across the contacts and that the nitric acid treatment mainly affected the network performance by modulating the contacts and reducing their contact resistances. Furthermore, a novel room-temperature press transfer technique was developed. This dry, ambient temperature deposition method allowed for the rapid and direct deposition of variable thickness SWCNT networks to a wide range of substrates, from flexible polymers to glass, silicon and metals. The developed process eliminates harsh and detrimental purification and dispersion steps, thus maintaining the high intrinsic performance of SWCNTs. Fabrication of novel freestanding SWCNT networks was also demonstrated. The freestanding SWCNT networks can be used for a wide range of novel applications. The aerosol-CVD synthesized SWCNTs were also demonstrated as flexible counter-electrodes in dye-sensitised solar cells. The SWCNT-network was combined with electrochemically deposited PEDOT, reaching comparable performance with standard platinum catalyst with energy conversion efficiencies of up to 4%. Fabrication and properties of hybrid materials consisting of SWCNT networks coated with amorphous carbon deposited by low energy plasma were studied. The carbon coating improved the mechanical durability of SWCNT films under nanoindentation and scratching.